Comments

Yes, just the extreme edge. It’s a hard character to describe, because all Peltigera are broadly concave, but the ones that are “turned down” have these rounded regions at the margins (especially at the lobe tips, not necessarily the lobe axils) that curl abruptly under in the last few millimeters from the edge.

You can see it best on the left side of the second photo (image 271020).

I think it has to do with the relative rates at which the upper and lower surfaces grow. If the upper surface grows faster, then it tends to curl under, if the lower surface grows faster, it tends to curl up. But since the overall shape is concave (it must continue to grow underneath long after the upper cortex stops growing?), in order for it to curl under at the edge, it needs to become scalloped and wavy. This results in the frilly appearance and the axils being erect.